The present invention relates to ultrafine cobalt metal powder consisting of fine crystallites, wherein the crystallites exhibit a habit ranging from rice-grain shaped to spherical and more than 90% of the crystallites have a diameter in the range of from 0.5 xcexcm to 2 xcexcm, a process for the production of the cobalt metal powder via the intermediate stage of the cobalt carbonate, and methods for the use of the cobalt metal powder and of the cobalt carbonate.
The main fields of application of ultrafine cobalt metal powder are the production of hard metals and of diamond tools. The two applications place different demands on the cobalt metal powder. For use in hard metals, a very low content of impurities such as sodium, calcium and sulphur is particularly important. It is also important that the content of oxygen and carbon is not too high. The particle size and particle shape are of secondary importance.
To produce hard metals, mainly mixtures of tungsten carbide and of about 6 to 15% of cobalt metal powder are sintered at temperatures of about 1350 to 1450xc2x0 C. This is a liquid phase sintering of the cobalt metal powder, during which part of the tungsten carbide dissolves in the cobalt. On cooling, recrystallization processes take place in the course of which small quantities of impurities such as sodium (Na), calcium (Ca) and sulphur (S) contained in the starting materials are already deposited preferentially at the grain boundaries of the tungsten carbide crystals. This can lead to a local reduction in strength and hence to a decrease in bending strength (12th International Plansee Seminar ""89, Vol. 2 (pages 421-428)). In the case of very fine hard metal parts such as, for example, microbores, this effect results in the tools readily breaking at the positions of decreased strength.
It is also important that the content of oxygen and carbon is not too high, with values to a total of up to 0.9 wt % being acceptable. Both an increased oxygen content and an increased carbon content can influence the carbon balance during the sintering process, so that the development of embrittlement through etaphasis or through formation of C-porosity owing to carbon esters may possibly result. The two effects also distinctly impair the quality of the hard metal.
In the production of diamond tools based on cobalt metal powder, which tools consist mainly of cobalt metal powder, synthetic diamonds and other powdered substances, for example, copper, tin, iron, nickel, etc. the influence of the physical properties such as particle size and particle shape quite definitely dominates. Although the chemical impurities in the above-mentioned elements can give rise to a microporosity, this is only a minor factor. The reason for this lies in the temperature range of 700 to 950xc2x0 C. conventionally employed in the production of diamond tools. In contrast to the production of hard metals, at these temperatures solid phase sintering occurs, so that the properties of the initial powders are predominantly preserved.
Experimentally it is observed that a decrease in the particle size gives rise to an increase in the hardness of hot-pressed cobalt segments. In general the Hall-Petch equation states that the hardness is reciprocal to the square root of the medium particle diameter.
This relation can be explained theoretically by the fact that the hardness is influenced by the specific proportion of grain boundaries per unit of volume, since the grain boundaries impede the propagation of dislocations. As the hardness correlates with the cutting properties of the segments, an increase in hardness frequently results in tools having a longer useful life and is therefore of great importance. In order to increase the specific proportion of grain boundaries per unit of volume, the primary particle size of the powders can be decreased (J. Konstanty and A. Busch in PMI, Vol. 23, No. 6, (1991)). Another possible method of increasing the specific proportion of grain boundaries per unit of volume consists however, at an identical or similar particle size, in altering the particle shape in such a way that the primary crystals have more of a rounded habit.
There are a number of different ultrafine cobalt metal powders which to varying degrees fulfil the requirements of manufacturers of hard metals or of diamond tools.
EP-A 0 113 281, owned by the firm Eurotungstene, Grenoble, France, describes the production of cobalt metal powder by the polyol process, whereby different cobalt compounds are reduced by polyols at 85xc2x0 C. This cobalt metal powder may contain up to 3 wt. % of carbon and oxygen, so that at the otherwise high chemical purity and the high specific proportion of grain boundaries per unit of volume, effected by the particle shape, an adverse influence on the hard metal properties cannot be excluded.
The commercial cobalt metal powder product Co UF from the firm Eurotungstene, according to technical information supplied by the company, is manufactured from cobalt hydroxides. This product is distinguished by a relatively high specific proportion of grain boundaries per unit of volume. However, the increased content of sodium and sulphur can be disadvantageous.
A completely different technical procedure is disclosed in U.S. Pat. No. 5,246,481, owned by the firm Sherritt Gordon, Alberta, Canada. Here the production of this powder is carried out through the reduction of cobaltammine sulphate solutions, to which have been added soluble silver salts as nucleating agents. The doping with silver salts can lead to exceptionally high contents of silver, typically of up to 3,600 ppm, in the cobalt metal powder. Furthermore the carbon content, which according to information from the company is about 1,750 ppm, is remarkable.
The principal object of the present invention is to provide a cobalt metal powder which does not possess the disadvantages of the powders described above.
There has now been found a cobalt metal powder which possesses the required properties. The present invention provides an ultrafine cobalt metal powder consisting of fine crystallites, wherein the crystallites exhibit a habit ranging from rice-grain shaped to spherical and more than 90% of the crystallites have a diameter in the range of from 0.5 xcexcm to 2 xcexcm, characterised in that it has a sodium content of less than 100 ppm and a carbon content of less than 500 ppm.
Preferably the content of sodium is less than 50 ppm and that of calcium and sulphur respectively is less than 30 ppm.
In an additional preferred embodiment, more than 90% of the crystallites have a length to width ratio in the range of from 1:1 to 5:1, while the diameter of the crystallites is preferably from 0.7 xcexcm to 1.1 xcexcm. The particle size of the crystallites, measured in accordance with ASTM B 330, is preferably from 0.7 xcexcm to 0.95 xcexcm.
Table 1 below provides a survey of the ultrafine cobalt metal powder according to the invention compared with various commercial products.
Compared with the commercial product Co IV C from HCST (Hermann C. Starck, GmbH and Co. KG, Goslar) the new product according to the invention exhibits a further increase in purity and again an increased specific proportion of grain boundaries per unit of volume due to a rice-grain shaped to spherical habit. In addition, in a preferred embodiment the product according to the invention has particle sizes of from 0.7 to 0.95 xcexcm, measured in accordance with ASTM B 330, whereas in the commercial product Co IV C these are larger than 0.95 xcexcm.
The present invention also provides a process for the production of the new cobalt metal powder. This is a process for the production of cobalt metal powder in which a soluble cobalt salt is reacted with a liquefied form such as a solution and/or suspension, of alkali carbonate, alkaline-earth carbonate, cobalt carbonate and/or ammonium carbonate and/or the respective hydrogen carbonates, in the pH range of from 5.5 to 6.8, the precipitate formed is separated off, washed with water until the required purity is attained, dried and the cobalt carbonate thus obtained is reduced to the cobalt metal powder.
A process for the production of cobalt metal powder via the intermediate stage of the cobalt carbonate is also disclosed in the Japanese patent application JP-A 78 123 722, owned by the firm Sumitomo, Japan. Owing to the processing conditions thereof, the said process leads to powders of a completely different morphology compared with that of the powders obtained by the process according to the invention.
The crystals obtained according to JP-A 78 123 722 preferably have a diameter of 1 to 2 xcexcm on average and a length equal to 10 to 20 times the diameter. This indicates, however, a low specific proportion of grain boundaries per unit of volume and consequently a lower hardness as well.
In the process according to JP-A 78 123 722, the reaction of the cobalt salts to form cobalt carbonate is carried out at pH values in the range of from 7.0 to 7.4. In reactions within this pH range the Co ions present in the solution are precipitated quantitatively. A disadvantage, however, is that there is a decline in the effectiveness with which the precipitate can be washed, which leads to the increased contents of Na, Ca and S in the end product.
In contrast, the process according to the invention is carried out with the pH value being adjusted to less than 6.8, preferably to 6.0 to 6.7. By this means a considerably more effective washing of the precipitate is achieved and hence the impurity content is definitely lowered. However, precipitation at a pH value of less than 6.8 is inconsistent with the requirement that the Co ions be completely reacted. The reaction solution flowing off is still pink-coloured, that is, it contains considerable quantities of unreacted Co ions, which would also enter the waste water. This portends on the one hand environmental problems and on the other hand, because of the high price of Co raw materials, an indefensible economic loss. In the process according to the invention, the precipitation is nevertheless carried out at a pH of less than 6.8. The grave disadvantages of this condition for the precipitation are avoided according to the invention by recirculating the cobalt contained in the reaction solution flowing off and by adding it to the CoCl2 solution prior to the reaction, if necessary with readjustment to a suitable pH value. There are a large number of feasible technical procedures which can be carried out in order to separate off cobalt as a sparingly soluble compound. These include:
precipitation of the cobalt as cobalt hydroxide by the addition of sodium hydroxide
precipitation of the cobalt as basic cobalt hydroxide by alkaline oxidation with hydrogen peroxide
precipitation of the cobalt as cobalt carbonate by the addition of alkali carbonates and/or alkaline earth carbonates or the respective hydrogen carbonates at suitable pH values
precipitation of the cobalt as basic cobalt carbonate by the blowing in of carbon dioxide.
By carrying out the precipitation according to the invention, the formation of a precipitate containing a particularly fine-grained cobalt is ensured. Measurements taken by means of the Malvern Master Sizer instrument on samples withdrawn from the precipitate-containing suspension showed that the D90 value is at most 90 xcexcm, particularly preferably at most 40 xcexcm. If the precipitations are carried out as in the known prior art, the d90 value is 130 xcexcm.
The reaction product is advantageously washed with water until the required purity is attained. Washing is carried out preferably in several stages, with the water temperatures selected being first of all in the range of from 0xc2x0 C. to 35xc2x0 C. and finally in the range of from 35xc2x0 C. up to boiling temperature. The cobalt carbonate is then dried, preferably in a moving bed. Finally the product is converted to cobalt oxide. The latter is reduced, preferably using hydrogen, to ultrafine, very pure cobalt metal powder.
It is particularly preferred that between the drying and reducing stages a calcination be carried out at temperatures of from 500xc2x0 C. to 800xc2x0 C. The actual reduction is advantageously carried out at temperatures of between 400xc2x0 C. and 550xc2x0 C.
The present invention also provides the use of the cobalt metal powder according to the invention as a binding material for diamond tools, hard metals and abrasive component parts.
This invention further provides the use of the cobalt carbonate obtainable by the process according to the invention for the production of cobalt oxide of the general formula CoO1xe2x88x92x for use in batteries. This cobalt carbonate is also eminently suitable as a doping agent for electroceramics.